When galvanizing steel in a conventional non-alloyed zinc bath, serious problems arise, when the steel contains more than 0.02 wt % of silicon: the resulting zinc coating is both too thick and too brittle and in addition it has a greyish aspect. This is due to the fact that the iron--zinc alloy layer, which forms on the surface of the steel when the latter is in contact with a conventional zinc bath, grows linearly with time during the entire duration of the immersion, when the steel contains more than 0.02 wt % of silicon. This is not the case with steels containing less silicon, as the growth rate is here proportional to the square root of the immersion time. The influence of the silicon content of the steel on the coating thickness is illustrated in FIG. 1, wherein the thickness peak on steels with 0.03-0.15 wt % Si is called the Sandelin peak.
Efforts have already been made in the past to cope with this problem. The Technigalva.RTM. process uses a zinc bath alloyed with 0.05-0.06 wt % of nickel. As shown in FIG. 1, the Sandelin peak disappears in the Technigalva.RTM. bath, but the coating thickness still increases with the silicon content of the steel. The Polygalva.RTM. process uses a zinc bath with 0.035-0.045 wt % of aluminium and 0.003-0.005 wt % of magnesium. As shown in FIG. 1, the Polygalva.RTM. bath gives rather good results; however it presents the drawback that its aluminium content has to be controlled very strictly, because the reaction between the steel and the bath blocks almost completely once the aluminium content of the bath exceeds 0.05 wt %.